In industrial technology, there are certain cases where a confinement building is required in order to isolate, from the outside world, certain installations which are dangerous owing to the fact that they are liable to emit pollutants, especially radioactive products, which, to comply with the safety standards, must not simply be discharged into the atmosphere. This is particularly true of nuclear reactors, to which reference will be made exclusively hereinafter, although it should be realised that this example is entirely non-restrictive and that this invention covers confinement enclosure in general for all installations capable of releasing dangerous products either temporarily or permanently.
Nuclear reactors are generally sited inside confinement enclosures designed to maintain a satisfactory level of leaktightness against radioactive products liable to escape from the primary circuit of the reactor during certain incidents, especially incidents which put the interior of the enclosure under pressure. In some constructions, this leaktight seal is obtained, in particular, by means of an impermeable membrane applied to the inside of the reinforced or prestressed concrete wall; in other constructions, it has been proposed to construct the enclosure with no membrane but with a double wall, with an intermediate space between the two walls serving to collect the gases, liquids and aerosols liable to pass through the inner or outer walls at a moderate rate of flow; the two walls are independent over their full height above ground level.
This arrangement ensures excellent confinement in the case of the accidents for which it is designed, but on the one hand it requires the use of special equipment at the moment when an accident occurs, which means that stringent precautions must be taken to ensure reliability, and on the other hand the arrangement does not make the best use of the materials as regards resistance to extreme loads. In fact, if the internal pressure increases to the point of causing cracking or fracture of the inner enclosure, the outer enclosure may be subjected to the full pressure within the confinement enclosure, and it will be appreciated that the resistance of the two independent walls in series is finally only as great as that of the stronger wall. For dynamic impact due to seismic shocks or impact from projectiles originating from inside or outside, better resistance is again generally obtained by using the entire mass of materials used in the construction of the walls to form a single thick enclosure.
From this point of view, confinement enclosures have already been proposed consisting of a single wall of reinforced concrete, more particularly prestressed concrete, characterised by the existence of a drainage network within the wall itself; this is the case particularly in Luxembourg patent 33.557 amd U.S. Pat. Nos. 3,320,969 and 3,778,948. In these known solutions, this drainage network usually consists of a set of holes in the form of parallel tubular channels produced during the casting of the concrete, with their openings arranged close enough together to ensure that any accidental crack will necessarily meet one of the channels or its porous surroundings, if any, thus enabling the pollutant phase to be drained off accordingly. The holes in the form of tubular channels are theoretically arranged in random directions inside the body of the wall, even though, for practical reasons, it is more convenient to make them vertical or horizontal; these two latter systems may, moreover, coexist, possibly comprising junction points. Sometimes there is advantageously provided within the wall of the enclosure a special permeable layer, e.g. of porous concrete, in which the drainage channels are located, thus increasing the efficiency of the arrangement; occasionally, also, inclusions of permeable strips consisting of tubes filled with gravel which connect the drainage channels of one of the horizontal or vertical systems to one another are provided during the casting of the concrete.
The known drainage networks provided in confinement enclosures are generally linked to a system for sucking escaped substances through external filters; the fluid circulating in the drainage network is usually gaseous and both the internal, possibly pollutant, gaseous phase and the outer air which would tend to pass through the wall of the enclosure which is assumed to be cracked following an accident occurring either internally or externally are drained into the said network, which has been put under vacuum by means of an extractor fan and a filter located outside the enclosure. This gaseous phase is filtered and then ejected into the outer atmosphere. However, in all these known systems, it is necessary to use suction systems or mechanical vacuum systems on which the safety of the installation depends, in the last analysis, and which cannot therefore ensure total reliability.